Satellite communication has become an important part of our overall global telecommunication infrastructure. Satellites are being used for business, entertainment, education, navigation, imaging and weather forecasting. As we rely more and more on satellite communication, it has also become more important to protect satellite communication from interference and piracy. There is now a demand from commercial satellite operators for satellite antennas that provide rejection of unwanted signals or minimise signal power to unwanted receivers.
Especially, satellite communication can be degraded or interrupted by interfering signals. Some interference is accidental and due to faulty ground equipment. Other interference is intentional and malicious. By directing a powerful signal at a satellite, the satellite can be jammed and prevented from receiving and retransmitting signals it was intended to receive and retransmit.
The above mentioned problems can be solved by creating a receive or transmit radiation pattern with zero or near-zero field strength, also known as a null, in the direction of the interfering signal or the unwanted receiver. Conventionally, a region of zero directivity or a null in a radiation pattern is produced by the summation of a main pattern having a wide flat gain distribution and a cancellation beam which is of the same amplitude but in antiphase with the main beam at the required location of zero field strength. It is known to use multiple feed elements carefully combined with the correct relative amplitude and phase to produce such cancellation.
Most commercial satellites these days use reflector antennas shaped to provide the desired regional coverage. The surface of the reflector in the reflector antenna can be modified during the design process using reflector profile synthesis software to produce the required beam pattern. An example of suitable reflector profile synthesis software is POS from Ticra. Reflector profile synthesis software of the type used in synthesising shaped reflectors for contoured beams can also be used to generate a pattern with low field strength in a predetermined direction. The reflector profile synthesis software numerically analyses the desired far field to suggest a surface profile of the reflector in order to create the desired beam. An example of a surface profile of a conventional reflector for producing a pattern with low field strength in a predetermined position is shown in FIG. 1. An example of a far field radiation pattern generated by a conventional reflector for producing a pattern with low field strength in a predetermined position is shown in FIG. 2. The min/max algorithms employed by conventional synthesis software to produce the appropriate surface profile rely on making smooth, differentiable changes to the surface and the resulting field, close to the zero, exhibits the typical quadratic behaviour of a cancellation beam approach. A problem with this approach is that quadratic cancellation patterns are sensitive to random surface errors of the reflector and to errors in the feed pattern as shown in FIGS. 8b and 9b. 
The invention aims to improve on the prior art.